Method and apparatus for recovery and refining of zinc

ABSTRACT

A method and apparatus for recovering and refining zinc from scrap metals having a high zinc content. The scrap metals are melted in a conventional furnace and introduced in a molten state into the distillation zone of the apparatus of the invention. The apparatus comprises an atmospherically sealed vertical column having a distillation zone and a communicating superposed fractionation zone. Arranged within the vertical column is a plurality of vertically spaced horizontally staggered trays constructed of a suitable refractory material. Molten material is continuously introduced into the apparatus through a metal seal and flows downwardly in a circuitous path over the trays disposed in the distillation zone. The metal in each tray is heated to temperatures sufficient to vaporize zinc by means of precisely controllable radiant heating elements located immediately above and below each tray. The zinc vapors rise through the distilling zone in countercurrent with the descending molten metal, pass through the factionating column where various impurities are removed, and then are directed into a condenser wherein they are condensed to metallic zinc of high purity.

llrown et al.

[ Dec. 11, 1973 METHOD AND APPARATUS FOR RECOVERY AND REFlNlNG OF ZINC[76] Inventors: Clayton R. Brown, 233 S. Hacienda Dr., Arcadia; MiltonTempler, 8013 San Lucas Dr., Whittier, both of Calif.

[22] Filed: July 13, 1971 [21] Appl. No.: 162,130

[52] US. Cl. 266/19, 75/88 [51] Int. Cl C22b 19/14 [58] Field of Search75/63, 88; 266/19, 266/15 [56] References Cited UNITED STATES PATENTS902,535 10/1908 Johnson 75/88 1,712,133 5/1929 Breyer 75/88 1,994,3493/1935 Ginder et al..... 75/88 1,994,356 3/1935 Pierce et a1 266/192,331,988 10/1943 Loevenstein 75/63 2,670,196 2/1954 Breyer et a1.266/19 3,045,995 7/1962 Handwerk 266/19 Primary ExaminerGerald A. DostAttorney-Whann & McManigal [57] ABSTRACT A method and apparatus forrecovering and refining zinc from scrap metals having a high zinccontent. The scrap metals are melted in a conventional furnace andintroduced in a molten state into the distillation zone of the apparatusof the invention. The apparatus comprises an atmospherically sealedvertical column having a distillation zone and a communicatingsuperposed fractionation zone. Arranged within the vertical column is aplurality of vertically spaced horizontally staggered trays constructedof a suitable refractory material. Molten material is continuouslyintroduced into the apparatus through a metal seal and flows downwardlyin a circuitous path over the trays disposed in the distillation zone.The metal in each tray is heated to temperatures sufficient to vaporizezinc by means of precisely controllable radiant heating elements locatedimmediately above and below each tray. The zinc vapors rise through thedistilling zone in countercurrent with the descending molten metal, passthrough the factionating column where various impurities are removed,and then are directed into a condenser wherein they are condensed tometallic zinc of high purity.

1 Claim, 5 Drawing Figures METHOD AND APPARATUS FOR RECOVERY ANDREFINING OF ZINC BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention relates generally to methods and apparatus for refiningzinc and more particularly to a method and apparatus for recovering andrefining zinc from zinc-bearing scrap materials.

2. Description of the Prior Art The rapid depletion of the earthsnatural zinc resources makes mandatory the development of economical,efficient, and ecologically acceptable methods for recovering zinc fromzinc-bearing scrap materials.

Zinc-rich materials are in relative abundance since numerous types ofshort useful life articles manufactured in large volume contain zinc incommercially recoverable quantities. The largest source of such materialis in the form of discarded zinc die-cast articles. Such articlesinclude automobile carburetors and fuel pumps, certain types ofhousehold applicance and office equipments, and aircraft forming dies.Unfortunately,'because of low efficiencies, high cost and atmosphericpollution problems inherent in presently known zinc recovery processes,much zinc-rich scrap is being lost to land fill dumps rather than beingcollected for recovery because of the low price being paid for suchscrap.

Of the numerous methods for recovering and refining zinc which aredisclosedin the prior art, the oldest and most common method is thebatch-type retort method which typically uses horizontal retorts formelting the scrap materials. In practicing this method retorts,typically constructed or graphite, silicon carbide or other refractorymaterials, are charged with the zinc-bearing material and are placed ina furnace wherein the material is melted by conduction heating. Whensufficiently high temperatures are reached in the retort the zinc in thescrap will vaporize and the zinc vapors will ascend into a spaceprovided within the retort above the molten metal. The zinc vapors arecontinuously evacuated from the retort and are diverted into a condenserwherein they are condensed to zinc metal or zinc dust, depending on thetype of condenser used.

Batch-type methods using horizontal retorts or muffle furnaces are quiteinefficient and present numerous processing difficulties. in a smallretort such as is commonly used in the prior art, the percentage of wallsurface in contact with the liquid material is very limited and becauseheating is by conduction through the walls of the retort, the efficiencyof vaporization of the zinc is extremely low. If the size of the retortis increased, numerous structural difficulties are encountered becauseof the very large hydrostatic pressures exerted by the heavy moltenmaterial. Additional problems encountered in typical retort-typeprocesses are attributable to the fact that the bath of molten metalwithin the retort is of necessity quite deep. Carrying out vaporizationin a deep bath of molten metal results in increased boiling points atthe bottom of the retort, and also results in violent ebullition, whichis objectionable because it contaminates the vapor above the liquid witha spray of impure metal. Retorts will quite often break while inoperation. This results in the molten zinc material flowing into thecombustion zone of the furnace and voluminous quantities of zinc oxideare then either released to the atmosphere or must be collected by verycostly air pollution equipment.

In methods such as the muffle furnaces all the heat for distillation isobtained by radiation from above the bath which is relatively deep, andirregular vaporization at the end of the cycle is experienced. Very lowfuel efficiencies are characteristic of this method.

Keeping an atmospheric seal between the combustion zone and thedistillation zone of the furance prevents problems because the seal isusually made of relatively small tiles of silicon carbide with theresultant numerous joints between the tiles, any of which may open.Leaks or breakage of the seal result in: (1) evolution of zinc oxidewith the same problems as related above in the description of retorts;and (2) oxidation of zinc vapors which can affect the quality of zincdust or result in lower recoveries of zinc if zinc is being produced.

Applicant is familiar with the following U.S. Pat.: Holstein, et al.Nos. 1,994,345, 1,994,346 and 1,994,358; Ginder Nos. 1,994,347 and1,994,348; Ginder, et al. Nos. 1,980,480, 1,994,349, 1,994,350,1,994,351, 1,994,352, 1,994,353, 1,994,354 and 1,994,357; Peirce, et al.Nos. 1,994,355 and 1,994,356; Langhorne No. 2,267,698, Poland No.2,463,468; Robson No. 2,473,304; and Lundevall No. 2,939,783.

' Some of the drawbacks of the batch-type retort or muffle furnacemethod are overcome by processes of the type illustrated by U.S. LettersPat. No. 2,939,783 issued to Lundevall. This patent relates to a processand apparatus for refining impure zinc from old diecast alloys and,unlike most retort processes, is substantially continuous employing ahorizontal furnace having a plurality of chambers interconnected byvarious passageways through which molten metals and vapors maycirculate. Processes of this type, since they are continuous, aresuperior to prior art retort or muffle furnace batch-type processes.

Another type of continuous process involving volatilizing of zinc metalis illustrated in U.S. Letters Pat. No. 1,994,355 and No. 1,994,356 toPeirce, et al. The invention disclosed in these patents relates to amethod and apparatus for volatilizing relatively pure zinc metal in asubstantially continuous operation. The apparatus of Pierce, et al. isvertically rather than horizontally disposed and comprises a chamberhaving a plurality of spaced trays adapted to hold pools of moltenmetal, each having an opening to permit downward flow of molten metaland upward flow of vapor.

The Peirce, et al., apparatus and method differ from that of ourinvention in several significant aspects. A major advantage of ourinvention over Peirce lies in the fact that in our invention the heatingand volatilizing of the zinc-rich metal are accomplished in a highlyefficient manner by radiation of heat from radiation elements located inclose proximity with the trays containing the molten metal. In thePeirce apparatus the heating is accomplished by a heating source outsidethe volatilizing chamber and comprising a massive furnace having a largeheating chamber which completely surrounds the volatilizing chamber.Another significant advantage of our invention relates to the fact thatbecause of the unique design of the apparatus we can advantageouslyprovide a plurality of precisely controllable temperature zones withinthe volatilizing chamber, thereby markedly increasing the efficiency andproductivity of the apparatus. Additionally, of great importance is thefact that the apparatus of our invention, being atmospherically sealed,in no way contributes to atmospheric pollution and, therefore, requiresno costly ancillary air pollution control equipment.

SUMMARY OF THE INVENTION The previously mentioned and otherdisadvantages of the prior art methods and apparatus for recovering andrefining of zinc from scrap metals make extremely desirable, from anoverall ecology standpoint, the provision of a method and apparatuswhereby such disadvantages may be avoided or overcome. It is animportant object of our invention, therefore, to provide a method andapparatus for the continuous recovery of zinc from scrap metals in whichthe apparatus is atmospherically sealed and economical to construct, andin which the method will produce high purity zinc from scrap metals withgreater efficiencies than heretofore possible.

It is a further object of our invention to privide an apparatus for therecovering and refining of zinc from scrap metals in which the processmay be carried out in a single vertical chamber housing both adistillation zone and a fractionation zone.

It is another object of our invention to provide an apparatus asdescribed in the preceding paragraph in which various preciselycontrollable temperature zones are provided within the distillation zoneso as to enable the maximum recovery of zinc from the scrap materials.

It is a further object of our invention to provide an apparatus asdescribed in which the zinc-bearing metal is contained in a plurality ofvertically spaced shallow trays and is heated to zinc volatilizationtemperatures by means of radiant heaters positioned both above and belowthe trays in the distillation zone and in close proximity therewith soas to efficiently volatilize the zinc.

It is another object of our invention to provide a method and apparatusof the type described in which the construction of the trays forcontaining the molten metal and the arrangement of the radiant heaterswith respect thereto are such that irregular boiling and superheating ofthe molten metal are precluded.

It is another object of our invention to provide a method and apparatusas described in the previous paragraphs in which the molten zinc-bearingmetal is caused to continuously cascade downwardly from tray to tray soas to encourage the release of zinc vapor from the molten metal, and inwhich the trays are constructed so as to encourage a circuitous flow ofthe metal through the trays so as to maintain impurities in liquidsuspension.

It is still another object of our invention to provide an apparatus aspreviously described in which the zinc vapors, as they rise in thevertical chamber, continuously encounter and mingle with the descendingmolten metal.

In summary, our invention comprises a method and apparatus forvolatilizing zinc metal in which molten metal containing zinc is passeddownwardly through an upright chamber having a distillation zone and asuperposed fractionation zone. A plurality of vertically spacedhorizontally staggered trays adapted to contain shallow pools of moltenmetal is positioned with the chamber. The molten metal in the trayswithin the distillation zone is heated to temperatures sufficient tovolatilize zinc by simultaneously radiantly heating the upper surface ofthe metal and the lower surface of the tray by means of radiant heatingelements positioned between the trays. The trays are positioned so as topermit the molten metal to cascade downwardly over the trays in acircuitous path and so as to permit the zinc vapor to rise to the top ofthe chamber where it is conducted to a condenser unit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional elevation of anapparatus for recovering and refining zinc from scrap metal inaccordance with the method of our invention.

FIG. 2 is a sectional view taken along line 22 of FIG. 1 illustratingthe construction of the metal seal for introducing the molten metal intothe apparatus and also illustrating the arrangement of the radiantheating elements in the distillation zone.

FIG. 3 is a plan view taken along line 33 of FIG. 1.

FIG. 4 is a fragmentary sectional view showing the relative positioningof the metal containing trays and the radiant heating elements, andillustrating the flow path of the molten metal from tray to tray.

FIG. 5 is a perspective view of one of the metal containing traysillustrating the details of its construction.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 and FIG. 2of the drawings, there is shown an upright atmospherically sealedstructure 12 having an internal chamber 14 in which the scrap materialis processed. Chamber 14 is divided into a lower distillation zone 16and a communicating upper fractionation zone 18. At the bottom ofchamber 14 is a residue catch basin 20 and a residue tapping means shownas orifice or tap hole 22 through which residue may be periodicallywithdrawn from the apparatus. At its top chamber 14 is provided with anoutlet 24 for withdrawing zinc vapor evolved therein. The outlet 24 isoperatively connected with a condenser (not shown) for condensing thezinc vapor. Depending upon the type of condenser used, zinc metal orzinc dust may be produced or the vapors may be oxidized to form zincoxide.

Located within chamber 14 above basin 20 is means for containing moltenmetal in the form of a plurality of vertically spaced trays 66 which areconstructed of graphite, silicon carbide or other suitable refractorymaterial. These trays are positioned in a horizontally staggeredrelationship and, as shown in FIGS. 4 and 5, have a base 30, an outerwall 32, an inner wall 34, and a pair of side walls 36. The walls arebeveled inwardly and, with the base, serve to retain shallow pools ofmolten metal. Inner wall 34 has a lesser wall thickness than the sideand outer walls as do ribs 38 and 40 which extend transversely of thetrays. The unique design and arrangement of the walls and connectingribs of trays provide means for encouraging the molten metal to follow acircuitous path as it circulates through the trays prior to cascading tothe tray immediately below. In this way impurities wihtin the moltenmetal are kept in liquid suspension. As can best be seen in FIG. 5, ribs38 and 40 have sections 38a and 400 which are further reduced in wallthickness and which are located in a staggered relationship to oneanother so as to encourage the circulation of molten metal within thetray along paths indicated by the arrows in FIG. 5. The trays areattached to structure 12 at their outer walls 32 and, as shown in FIGS.1 and 4, cantilever into chamber 14.

Near the top of distillation zone 16 there is provided a charging means42 for accommodating the introduction into the apparatus of moltenzinc-rich scrap material from a conventional melting furnace (notshown). Charging means 42, as can best be seen in FIG. 2, is in the formof a metal seal arrangement in which a downwardly inclined passageway 44through an outer wall of structure 14 provides means whereby moltenmetal can be introduced into the apparatus. Passageway 44 is positionedto communicate with means for maintaining a pool of liquid metal withinthe chamber shown in the form of an upwardly inclined member 46 disposedinternally of chamber 14. As metal is poured into the apparatus, thepool of molten metal formed between the chamber walls and member 46seals passageway 44 from atmosphere so that vapors from chamber 14cannot escape.

Referring particularly to FIGS. 2, 3 and 4, means for heating the metal,contained within the trays located in distillation chamber 16 areprovided in the form of radiant heating elements 50 which are positionedimmediately above and below each tray. Heating elements 50 areconstructed of a suitable refractory material such as silicon carbide orgraphite and, in the preferred form of our invention, heating isaccomplished electrically by using electrical resistance rods such asgraphite resistor rods. If desired, however, the heating elements may bemade of tubular construction and be adapted for internal burning ofcombustible fuels such as oil or gas.

Heating elements 50 extend across and sealably through structure 12, asindicated in FIG. 2, and have their ends connected to common manifoldsor electrical power lines 51. For purposes of illustration, we show twobanks of heating elements which are adapted to be separately controlled.It is to be appreciated that any number of separately controlled-banksof heating elements greater than one can be used so as to provide aplurality of different controllable temperature zones withindistillation chamber 16.

Means for measuring the temperatures at different locations within thedistillation zone are provided in the form of pyrometers 52 which extendthrough the walls of the processing structure into the distillationzone. These pyrometers may be connected with controls (not shown) forautomatically regulating the energy output of the banks of heatingelements within the distillation zone in a manner well known in theprior art.

OPERATION FIG. 5, and will then flow over inner wall 34a onto tray 26b,which is the first tray in the distillation zone. The circulationpatterns of the molten metal within the trays, which result from theunique design of the circulation means within the trays, serve to keepresidue materials in solution and prevent residue buildup on the trays.As molten metal is added to the apparatus either continuously or atfrequent intervals, the molten metal in the trays will continue tocirculate and overflow so as to cascade downwardly in a circuitous pathfrom tray to tray.

Important features of our invention include the unique design of themetal containing trays and the radiant heating of the metal containedtherein by means of radiant heating elements strategically placedimmediately above and below each tray in the distillation zone. Becauseof the shallowness of the trays and relatively large surface area of themolten metal which is exposed to heat both at its upper surface from theupper heater and at its lower surface from the bottom of the tray whichis efficiently heated by the lower heater, superheating is avoided anduniform and extremely efficient volatilization of the zinc takes place.In practicing our invention, molten metal is added to the apparatus at arate so as to maintain shallow pools of molten metal in each tray, andso that a small amount of metal continuously flows into catch basin 20.Impurities less volatile than zinc such as highaluminum content residuesare periodically removed from the apparatus through tap hole 22.

Another important feature of our invention relates to the novel andunique means provided for maintaining precisely controllable temperaturezones within the apparatus. By controlling the energy radiated from theheating elements in the manner previously described, temperature zoneswithin the distillation zone can be precisely maintained and controlledso as to maximize the vaporization of the zinc as the molten metal flowsdownwardly through the apparatus, thereby maximizing-the percentage ofzinc which can be recovered from the scrap.

As the metal cascades from tray to tray, zinc is constantly vaporizing.Because of the constant breaking of the surface of the molten metal, thevapors are relased more readily than n a static bath and in iseffectively precluded.

As the zinc vapors rise through the distillation zone, they tend to bepurified through a washing action resulting from their comingling withthe descending molten metal. As the vapors rise above the distillationzone they pass directly into the fractionation zone as indicated by thearrows 60, wherein lead, iron, tin, and aluminum are removed in a mannerwhich is well known in the art and, consequently, will not be describedin detail herein. If removal of cadmium is desired, the zinc vapors canbe passed through a separate column wherein cadmium may also be removedin a well known manner.

Within the fractionation zone which accepts the constant flow of zincvapors from the distillation zone, we

position trays constructed and arranged in a manner identical to thosein the distillation zone. As a result of reflux action within thefractionation zone, molten metal will of course move downwardly fromtray to tray and will pass into the distillation zone for reheating andvaporization. If desired, removable insualting walls or insulatingcurtains (not shown) canbe provided within the fractionation column tocontrol the reflux action in a manner well known in the prior art.

Referring to FIG. 1, we provide at the upper end of fractionation zone18 a sensing means 54, which may be in the form of a pressure-sensingdevice, for sensing the pressure of zinc vapors within the apparatus.Sensing means 54 is connected to control instrumentation (not shown)which is adapted to override the controls which regulate the heatingelements within the distillation zone should the rate of distillationbecome too great for the condenser to properly function.

The zinc vapors reaching the top of the fractionation zOne pass throughpassageway 24 into the condenser for condensation to form metallic zincor zinc dust. If metallic zinc is produced, it can be fed directly intoan alloying furnace for conversion into die-cast alloy.

We claim:

1. An apparatus for recovering zinc metal from zinc containing scrapmetal comprising:

a. a single atmospherically sealed upright chamber having a distillationzone and a communicating superposed fractionation zone;

b. a plurality of trays disposed within said chamber in avertically-spaced, horizontally staggered relationship, said trays beingadapted to contain a shallow pool of liquid metal and comprising a base,an inner wall and outer wall, side walls interconnecting said inner andouter walls, and a plurality of transversely extending ribs havingspaced sections of decreased wall thickness, said trays being arrangedso as to allow an upward flow of vapor through said upright chamber anda downward cascading of liquid metal from tray to tray through saidupright chamber;

. heat radiating means located above and below d. control means forcontrolling said heat radiating means so that a plurality of temperaturezones may be established and maintained within said distillation zone;

e. metal seal means for introducing liquid scrap metal into said uprightchamber without exposing said chamber to atmosphere; and

f. means for sensing metal vapor content within said fractionation zoneand for overriding said control means when the metal vapor contentexceeds a predetermined level.

